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Investments - Additional Information (Detail) - USD ($)		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses			$ 9,414,000
Gross unrealized losses for 12 months or greater	[1]		$ 1,843,000
Insurance enhanced asset-backed, commercial mortgage-backed, and credit securities		$ 33,000,000
Investments in insurance enhanced municipal bonds		1,136,000
Ratings without insurance		0
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		$ 31,900,000
Asset backed and taxable municipal bonds as a percentage of total cash and invested assets		2.10%
Maximum
Schedule of Available-for-sale Securities [Line Items]
Investments in a single issuer as a percentage of shareholders' equity		6.00%	5.00%
Pre-Refunded Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds		$ 0
Municipal Bond Insurance Association
Schedule of Available-for-sale Securities [Line Items]
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		6,300,000
Assured Guaranty Corporation
Schedule of Available-for-sale Securities [Line Items]
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		15,400,000
Taxable Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		21,600,000
Federal Home Loan Mortgage Corporation
Schedule of Available-for-sale Securities [Line Items]
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		10,200,000
AA Rating
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds		$ 1,100,000
Insurance enhanced municipal bonds as a percentage of total cash and invested assets		0.10%
One of the Company's variable interest VIE's, invests in distressed securities and assets
Schedule of Available-for-sale Securities [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		$ 22,900,000	$ 26,300,000
Variable interest entities, maximum exposure to loss		37,200,000	40,500,000
Second VIE that provides financing for middle market companies
Schedule of Available-for-sale Securities [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		36,400,000	33,800,000
Variable interest entities, maximum exposure to loss		41,600,000	43,800,000
New limited partnership that invests in distressed securities and assets and considered a VIE
Schedule of Available-for-sale Securities [Line Items]
Significant variable interest in fair value of the non-consolidated VIE		22,900,000	17,800,000
Variable interest entities, maximum exposure to loss		52,100,000	$ 51,300,000
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		$ 1,204,000
Weighted average credit enhancement		23.90%
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		$ 100,000
Asset-backed Securities | AA Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		46,000
U.S. Treasury and Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		2,060,000
U.S. Treasury and Agency Obligations | AA+ Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		247,000
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		601,000
Obligations of States and Political Subdivisions | Investment Grade Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		147,000
Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		3,511,000
Mortgage Backed Securities | AA+ Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		$ 58,000
Percentage of unrealized losses for 12 months or greater		97.30%
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		$ 3,554,000
Weighted average credit enhancement		28.40%
Investments in asset-backed, commercial mortgage-backed securities and taxable municipal bonds		$ 10,200,000
Commercial Mortgage-Backed Securities | AA+ Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		788,000
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		7,424,000
Corporate Bonds | Investment Grade Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		995,000
Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses		2,296,000
Foreign Corporate Bonds | Investment Grade Rating
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses for 12 months or greater		$ 345,000
Percentage of unrealized losses for 12 months or greater		78.30%
[1] Fixed maturities in a gross unrealized loss position for twelve months or longer are primarily comprised of non-credit losses on investment grade securities where management does not intend to sell, and it is more likely than not that the Company will not be forced to sell the security before recovery. The Company has analyzed these securities and has determined that they are not other than temporarily impaired.

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